1. Field of the Invention:
This invention relates to devices for sensing light and developing electrical signals representative of the received light, and more particularly to devices for sensing light of a light source as reflected by an object and providing an electrical readout necessary for focusing a focusing apparatus.
2. Description of the Prior Art:
A focusing apparatus utilizes a plurality of photo-electric transducer elements responsive to the reflection of radiation from a light source from an object for producing electrical signals. Known systems include circuits for computing the differences in signals between two successive elements. A determination of either the distance from the apparatus to the object or the focusing condition of an objective lens is based on these computed differences. Such systems are shown, for example, in U.S. Pat. Nos. 4,032,934 (issued June 28, 1977), 4,268,137 (issued May 19, 1981) and 4,357,085 (issued Nov. 2, 1982). In this type of apparatus, however, the accuracy of determination rapidly decreases for distant objects, and the ambient light causes the photo-electric transducer elements to produce spurious noise signals which may mask actual signals at a low level. It is, therefore, of great importance to solve such problems.
Attempts have been made to take into account these problems, for example, as proposed in U.S. Pat. No. 4,315,159 (issued Feb. 9, 1982). This light responsive device provides an electrical readout by using two signal storing circuits for every one photo-electric transducer element. The signal storing circuits are arranged so that those photo signals from the transducer element generated when the light source is energized, that is, photo signals of the metering light plus ambient light or complex light dependent signals, are accumulated on one or the two storing circuits. Accumulated on the other storing circuits are other photo signals, the ambient light dependent signals, generated when the light source is not energized. During a certain integration period, the output signals from the storing circuits are computed to obtain information necessary for focusing.
In this prior known light responsive electrical readout device the complex and ambient photo signals are stored in different circuits. The signal storing characteristic is not always uniform from circuit-to-circuit. The object distance is evaluated by subtracting one of the outputs of the storing circuits from the other. Accordingly, it becomes impossible to remove only the influence of the ambient light. Therefore, information responsive only to the metering light cannot be read out. This lowers the discrimination ability of the focusing apparatus.